1. Field of the Invention
The present invention relates to a multi-layer structure having excellent gas-barrier properties, and more particularly to a multi-layer structure having a good fabricability such as deep drawing ability, less malodor and discoloration and excellent transparency, heat resistance and gas-barrier properties.
2. Description of the Prior Art
Polyamides produced by the polycondensation of xylylenediamine and an aliphatic dicarboxylic acid, for example, polyamides produced from m-xylylenediamine and adipic acid (hereinafter referred to merely as “nylon MXD6”) have been extensively used as gas-barrier packaging materials such as films and bottles because of their high strength, high elastic modulus and low permeability to gaseous substances such as oxygen and carbon dioxide.
The nylon MXD6 exhibits a good heat stability upon melting as compared to other gas-barrier resins, and therefore, can be co-extruded or co-injection-molded with other thermoplastic resins such as polyethylene terephthalate (hereinafter refer to as merely “PET”), nylon 6 and polypropylene. Therefore, the nylon MXD6 is recently increasingly used as a gas-barrier layer of a multi-layer structure.
Usually, the multi-layer structure composed of a thermoplastic resin is heated to a temperature not less than the softening point or glass transition point of the resin constituting the multi-layer structure to fabricate it into containers, etc., by deep draw forming or blow molding. However, in the case of crystallizable thermoplastic resins, crystallization occurs at the glass transition point or higher temperatures, and the crystallization rate increases with the temperature rise until reaching a certain temperature. Therefore, because of the crystallization, the thermoforming may become impossible in the fabrication which requires high temperatures or prolonged preheating time.
A gas-barrier multi-layer container having a polyolefin layer made of polyethylene, polypropylene, etc., and a nylon MXD6 layer is proposed in Japanese Patent Publication No. 56-23792. However, the polyolefin used in the above multi-layer structure has a softening point usually higher than the glass transition point of nylon MXD6, and therefore, an adequate temperature for the fabrication is much higher than the glass transition point of nylon MXD6, thereby making the crystallization easy to occur. In particular, at around an adequate temperature for the fabrication of polypropylene, usually near 160° C., the crystallization rate of nylon MXD6 becomes very high. If the fabrication of the multi-layer structure is conducted in the highly crystallized state, the nylon MXD6 layer suffers from uneven thickness and whitening, thereby failing to obtain shaped articles that are practically satisfactory in shape and properties such as transparency. Therefore, the multi-layer structure having the above construction has been conventionally fabricated by in-line molding method in which the multi-layer structure immediately after melt-extrusion is fabricated at a temperature higher than the crystallization temperature, or under limited conditions in which the multi-layer structure is rapidly preheated for a short period of time so as to prevent the crystallization.
To improve the fabricability, etc., Japanese Patent Application Laid-Open No. 1-141737 proposes a multi-layer structure comprising a layer made of a mixture of nylon MXD6 and a hardly-crystallizable or amorphous polyamide resin and a layer made of another thermoplastic resin. However, since the polyamide mixed with nylon MXD6 shows a lower gas-barrier property than that of nylon MXD6, the resultant multi-layer structure has low gas-barrier properties. Thus, it has been difficult to obtain a multi-layer structure fully satisfying both the fabricability and the gas-barrier properties.
A crystallizable thermoplastic resin is usually processed at its melting point or higher temperatures. When a polyolefin and a higher-melting crystallizable polyamide are co-extruded into a multi-layer structure, a polyolefin resin layer and a polyamide resin layer both in a molten state are contacted with each other in a feed block. As a result, the polyolefin resin is subject to a severe heat history. In particular, in the production of a multi-layer structure containing polypropylene, discoloration, malodor generation, etc. due to heat degradation of the polypropylene resin unfavorably occur.
The glass transition point of the nylon MXD6 is usually about 85° C., but lowered upon the absorption of water. For example, the multi-layer structure comprising a nylon MXD6 layer and a PET layer which is produced by a stretch blow molding shrinks drastically when immersed in a hot water of 80° C. or higher. Therefore, when a multi-layer structure containing nylon MXD6 is applied to containers for foodstuffs or beverages which are subjected to a hot water treatment or a high-temperature filling for sterilization, it is necessary to subject the multi-layer structure to treatments accompanied by crystallization such as stretch heat-setting.
As described above, it has been demanded to develop a multi-layer structure comprising a combination of nylon MXD6 and another thermoplastic resin which is excellent in the fabricability such as deep drawing ability and stretch blowing ability, and is capable of providing containers that are fully satisfactory in transparency, heat resistance, appearance and gas-barrier properties.